Optical spectrum analysis of coherent 1-D signals scanned in a TV format is accepted as the most powerful technique for the processing of large time-bandwidth products. From recent developments, it has been expressed as an analog implementation of the FFT. Nevertheless, in its usual analysis as a ‘folded spectrum’, little attention has been paid to the effects of the 2-D raster on the coherence of the signal, which the present study shows to be of great importance. From experimental data, a new theoretical approach is derived, which suggests drastic limitations for the spectrum analysis of wide-bandwidth signals.